The value of an RF sensor; that is, a device that measures the RF current and voltage supplied to the plasma processing chamber of a microelectronics processing tool by a high power RF supply, is well established. Numerous patents, such as, for example, U.S. Pat. No. 6,501,285, have been granted for design and application of both the sensor and associated signal processing electronics. However, each of these inventions has focused on maximizing the electromagnetic performance of such systems. The fact that nearly all RF sensor applications involve retrofitting of the sensor to an existing process chamber that was not originally designed to accommodate the device has largely been overlooked. As a result of this performance focus, existing RF sensors are too large to fit in most applications without extensive and undesirable modifications to the processing tool and/or the RF sensor.
A typical RF sensor 10, as depicted in FIG. 1, comprises a short piece of coaxial transmission line, shielded pickups, and passive or active filtering circuitry. The RF sensor 10 itself forms the coaxial transmission line in the following manner. First, the sensor case or box 14 forms the outer conductor and shielding for the pickups. A constant diameter solid rod made from silver plated copper or other similar material forms a center conductor 18. Dielectric material 22 consisting of silicon, quartz, silicon carbide and/or alumina, among other materials, are used to maintain a predetermined geometric relationship between the center conductor 18 and the ground plane. A capacitive voltage pickup 26 and an inductive current pickup 30 are placed within the dielectric material 22. A bulkhead connection 34 feeds the leads from these pickups 26, 30 through the outer conductor to corresponding filter circuits 38, along with possible additional signal processing circuitry. The modified signals are then sent to an additional unit (not shown) for digitization.
The above configuration of a typical RF sensor has significant performance advantages. First and by forming a length of transmission line as shown in FIG. 1, the RF sensor assures that the pickups are exposed to a uniform electromagnetic field, regardless of the surrounding geometry. As a result, pickups have a constant gain as a function of current and voltage, independent of their application. Second and by forming a shielded enclosure, the RF sensor assures that the pickups are only sensitive to fields due to current on the center conductor and potential difference between the center and outer conductor, and not sensitive to extraneous fields. Finally, the sensor configuration easily accommodates standard RF connectors, and thus allows calibration on a test stand. Because of the sensor's design, this calibration is maintained independent of application.
Standard inductive current pickups and capacitive voltage pickups have increasing gain with increasing RF frequency, as illustrated graphically in plot 42 depicted in FIG. 2. This illustrated feature has the drawback of increasing the dynamic range of signal magnitude that must be accurately digitized by the RF sensor electronics. The simplest known way to correct for the increasing gain is to incorporate active or passive filtering in the sensor, as shown in FIG. 1, thereby resulting in a flatter response with frequency, as depicted according to plot 46, FIG. 2. Incorporating filtering in the sensor avoids the complication of a length of transmission line between the pickups and the filters. Due to transmission line effects, the total gain of the circuit can vary in unexpected ways with frequency.
The filtering circuit can also be used to maximize the signal from the pickups, allowing their size to be minimized. Minimum size is essential for minimizing stray impedances that result in crosstalk. More specifically, any inductance in the capacitive pickup results in current level impacting the voltage signal, and any capacitance on the inductive pickup results in voltage level impacting the current signal.
Finally, incorporating signal processing, as is done in typical RF sensor, has further performance advantages. The voltages generated by the pickups are orders of magnitude lower than the levels that are found in the plasma tool. Signal processing in the sensor, such as mixing to an intermediate frequency (IF) or even complete digitization greatly reduces the risk of the signal from the pickups being corrupted before they are quantified.
A standard RF sensor is very easily mated to a standard transmission line using standard RF connectors. However, this option is rarely, if ever, available. Rather, the RF sensor must be retrofitted to the existing RF power path in the plasma processing tool. This path typically consists of a center conductor of varying dimensions, an air dielectric, and a poorly defined ground plane. Installation of an RF sensor in these conditions requires modification to the existing power path. In addition to requiring significant effort and application-specific parts, these modifications can result in unacceptable changes to the electromagnetic characteristics of the power path.
Yet a further difficulty results from the physical size of the sensor device; wherein such devices are usually defined by a cubic box-like configuration in which each side of the configuration is several inches in length and width. In many cases, space for installation is simply not available. This results in one of two undesirable solutions. First, extensive modifications, such as in the form of spacers, additional conductors and custom fittings are required. Installation of these extensive modifications is costly and time-consuming and may also alter tool performance. Alternatively, and rather than mounting the RF sensor in proximity to the plasma chamber where it can most effectively monitor the process, the sensor is placed in a roomier, but more remote location, where its performance is impaired.
In conclusion, typical RF sensors have compromised usability for performance. The standard design is readily calibrated and assures that readings on a process chamber are the same as those obtained on a test stand. This performance is achieved at the cost of significant modification of the electrical or power path of the process chamber. This modification is costly and time consuming, and can significantly compromise the electromagnetic characteristics of the tool and impact processing performance.
Achieving a low-performance miniature RF sensor is relatively straightforward. As long as a capacitor and inductor are placed in proximity to the center conductor, signals that are roughly proportional to voltage and current will be generated. The challenge, however, is in maintaining high performance in a miniature RF sensor. In order to obtain high performance, practitioners of the art must successfully maintain gain, directivity, and isolation within a miniature sensor package. To date, Applicant is not aware of a miniature RF sensor package that accomplishes these objectives.
Therefore and according to a first aspect, there is provided a miniature RF sensor for measuring the RF current and voltage supplied to a plasma processing chamber of a microelectronics processing tool by a high power RF power supply, the assembly comprising a sensor head and including a conductor that is formed as one side of a box-like structure forming a housing, said structure including a current pickup and a voltage pickup, each of said pickups being stacked in relation to one another and said conductor.
In one version, the current and voltage pickups are provided in a printed circuit board construction, wherein the voltage pickup is formed from a grid or mesh element disposed in spaced relation relative to said conductor, in which said conductor can be a strap conductor of a plasma tool. The PCB construction, quarter wave transforming filter, stacked pickups, triaxial shielding, and skin-effect filtering, each feature resulting in a high performance miniature RF sensor. The proposed design meets both retrofitting and electromagnetic performance goals.